He “won’t release his tax returns, won’t reveal his bundlers, left Massachusetts with the hard drives from his computer. He believes in keeping the public in the dark,” Axelrod said in an interview with ABC News Radio behind the scenes at the first stop on Obama’s campaign bus tour through Ohio.

Axelrod said Romney’s portfolio of offshore investments -- which include a Bermuda-based company, funds in the Cayman Islands and a Swiss bank account -- raises what he called a “Bermuda Triangle” of financial questions.

“Just what were the benefits there? Did he actually pay taxes in some years? We don’t know,” he said.

“Why would you transfer your Bermuda business…to your wife the day before you became governor? Why did you not want that on your disclosure form?” he questioned later, referring to media reports that have scrutinized the holding.

Asked directly whether he is accusing Romney of tax cheating, Axelrod said no.

“The only reason to have these accounts is to shelter your taxes,” he said. “The question is, exactly what did he pay, and I think that he -- there’s only one way to answer that question: if you don’t have anything to hide why not just release that information? And Gov. Romney really doesn’t have a good answer for that.”

In January, Romney released his 2010 tax return amid intense public scrutiny and pressure from his GOP primary rivals. He has declined to release returns from additional years.

The Romney campaign did not immediately respond to ABC News’ request for comment.

(NEW YORK) -- Voters in 10 states are making their picks for the Republican presidential nomination on Tuesday, the biggest single day of contests in the race.

Below, watch live streaming video coverage of Super Tuesday, featuring the political teams of ABC News and Yahoo! This coverage will be anchored by ABC News political director Amy Walter and Yahoo!'s David Chalian. They will be joined throughout the night by Diane Sawyer, George Stephanopoulos, and others. ABC News Radio’s coverage, anchored by Aaron Katersky from ABC News headquarters in New York, will be a part of this special digital coverage.

NOTE: Coverage begins at 7 p.m. Eastern and is expected to run until at least 11 p.m. ET.